1. Field of the Invention
The present invention relates to measurement method of the rate of transmission of a vapor through a sample, such as a film of a polymer, a ceramic or a composite or a biological sample.
2. Description of the Related Art
Understanding of the transmission of gasses and vapors through materials of great importance in many fields, for example in medicine, food packaging and for chemical separation processes. Various techniques for measuring the rate of transmission of gasses and through a film are known. Some techniques are based on electrochemical principles, others use a sensor such as a modulated infra-red (IR) sensor. Other techniques measure the pressure increase caused by permeating gasses or vapors, for example using an ion gauge or a PIRANI gauge.
For example, a common known device for measuring rates of transmission of water vapor is the device sold under the Trade Mark PERMATRAN, which uses a modulated IR sensor. This known device uses samples having an area of the order of 100 cm2 and has a practical resolution of around 5 mg/m2/day. However, it would be desirable to obtain a greater sensitivity to allow the measurement of lower absolute rates of transmission of vapor. This would allow, for example, the study of samples with a given transmission rate per unit area having a smaller effective area or samples having a lower transmission rate per unit area, than is possible with the known IR sensor devices.
It is also known to measure the rate of transmission of various gases through a sample using a mass spectrometer to measure the partial pressure of the various species of gas molecule transmitted from a gas chamber through the sample into a vacuum chamber under vacuum. In some known mass spectrometer techniques, the partial pressure in the vacuum chamber decays exponentially over time. This creates difficulties in measuring gas transmission rates, because of difficulties in both (i) modeling the decay to relate the decay to a transmission rate, and (ii) accurately extracting the decay co-efficient from the data, especially at low absolute transmission rates. This is a particular problem for larger molecules which exhibit exponential behavior, that is the measured particular pressure is the sum of two different exponential decay curves having a different magnitudes and time constants. This even further complicates any model used to extract coefficients.
There are also known mass spectrometer techniques in which the test sample is fixed in the apparatus separating the gas chamber from the vacuum chamber and the gas chamber is supplied with gas at a constant pressure from an external source. However, such an apparatus presents a number of practical difficulties. It is necessary to have a pressurized source of the gas. It is cumbersome to change the test sample. Also, it is impractical to use the apparatus at Ultra High Vacuum, because the evacuation process including baking risks damage or destruction of the test sample. Consequently the sensitivity to low absolute transmission rates is reduced because the background measurements are high.